This invention relates to high definition television. More particularly, this invention relates to a method and apparatus for encoding and decoding video signals for HDTV.
It is widely anticipated that some form of high definition television (HDTV) will be the next major advance in TV broadcasting. By "high definition" we simply mean that images will be transmitted with much greater resolution than is currently done. The current thinking is that HDTV in the United States will offer approximately six times as much information per frame than is currently transmitted. If no changes are made in the manner of forming the television signals, that would mean, of course, a six fold increase in the bandwidth required for transmitting the HDTV signal.
Conventional Television signals that are broadcast terrestrially occupy government-specified frequency channels. There is essentially no unused frequency bandwidth that can be devoted to HDTV signals except, perhaps, for the guard bands that are currently provided between adjacent TV channels. The guard bands are included in the spectrum allocation scheme to reduce the potential interference between the signals of adjacent channels. The interference might occur by virtue of unavoidable transmission into frequency bands outside the band specified for the transmitter, and/or undue reception by the receiver of signals from outside the band specified for the receiver. The challenge for HDTV designers, therefore, is to compress the HDTV signal, to communicate the compressed signal to a receiver within the available bandwidth, and to uncompress the signal within the receiver and restore to its original form, without undue degradation.
A number of techniques have been proposed in the art for reducing the bandwidth of TV images. Typically, these techniques reduce bandwidth by encoding the video signals, identifying redundancies in the encoded signals, and extracting the redundant signals in such a way that the original signal can be reconstituted by the receiver. Different techniques for identifying redundancies are used in the art. Some are better suited for still images, while others are better suited for moving images.
One technique for reducing the bandwidth of moving images is generally referred to as motion compensated predictive coding. A comprehensive description of an arrangement for terrestrial transmission of HDTV signals, using motion compensated predictive coding, is found in a copending application, titled "Noise Immunity in HDTV Signals", Ser. No. 07/532,526, which was filed on Jun. 1, 1990. The following summarizes the principles employed in above-identified application.
The image provided by the HDTV camera is first compressed by employing temporal information. Specifically, each frame is subtracted from a prediction made by displacing bocks in the immediately previous frame according to a motion vector field derived by comparing the current frame with the immediately previous frame. The difference between the current frame and the prediction--the "difference frame"--is encoded by converting the signal into the transform domain in both horizontal and vertical dimensions. The frequency domain signals corresponding to transform blocks are grouped in clusters. A vector pattern represents the transform coefficients that are kept and discarded for all the blocks in a cluster. The retained coefficient values for each cluster are transmitted as analog signals, while the vector patterns identifying them as a particular set of retained values are transmitted digitally during the vertical retrace interval. A vector quantization technique is used to reduce the amount of digital information required to code these keep/discard patterns.
Transmitting digital signals presents problems that are qualitatively different from the problems that are encountered when transmitting analog signals. The primary difference lies in the heightened sensitivity of digital transmission to small variations in signal-to-noise ratio.
This phenomenon--which is sometimes referred to as the "threshold effect"--can be illustrated by considering the case of two television receivers that are respectively located at 50 and 63 miles from a television broadcast station. Since the power of the broadcast signal varies roughly as the inverse square of the distance, it is easily verified that the difference in the amount of signal power received by the television receivers is about 2 dB. When a digital transmission scheme is used, it is quite possible that the receiver that is 50 miles away from the transmitter will exhibits a bit-error rate of 10.sup.-6. If the 2 dB of additional signal loss for the more distant TV set translates into a 2 dB decrease of the SNR at the input of the receiver, then the receiver that is 63 miles away from the transmitter will operate with a bit-error rate of about 10.sup.-4. With these bit-error rates, the TV set that is 50 miles away would have a very good reception, whereas the TV set that is 63 miles away would probably have a very poor reception. This relatively sharp degradation in performance over short distances is generally not considered acceptable by the broadcasting industry.
By comparison, the degradation in performance of presently used analog TV transmission schemes is much more gradual. Noise at the receiver translates to "snow" on the image, and the "snow" simply increases as the quality of the reception decreases, but the image itself is essentially not affected. The noise merely adds to the signal; it does not dramatically alter it. Although, as the amount of "snow" increases, the effective resolution of the image decreases because the eye tries to integrate the signal in an effort to reduce the objectionable effects of the noise. The only time when noise is truly objectionable is when, with certain types of noise sources (such as noise due to echos), an artifact image is produced.
Because the noise in analog transmission of TV signals generally manifests itself in relatively unobjectionable ways, and because viewers are already used to that form of loss in quality, it is the object of this invention to create an arrangement for digital transmission where the degradation due to poor reception is graceful; and the characteristics of the degradation are akin to the degradation characteristics of analog signals.